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Prime Minister Bill English has responded to having his personality compared to that of a rock by Labour's deputy leader - saying their new positive approach under Jacinda Ardern "lasted about three days".

"And the reason is because Labour is still Labour. They have always had a pretty negative, limited view about what can be achieved in New Zealand, we have a positive view," English told media after his party's transport announcement in Auckland.

Ardern again said she wanted her campaign to reflect "relentless optimism", but Davis was happy to take some pot shots at National.

"Everything has changed. The vibe has changed. The momentum has changed. The energy has changed. The election has changed," Davis, the MP for Te Tai Tokerau, said.

"We all know about the Jacinda effect - pure optimism. When we look over at the other side there and we see a Prime Minister with the personality of a rock.

"We have got Jonathan Coleman the doctor of death, we have got Steven Joyce who is as authentic as a $4 Rolex, we've got Gerry Brownlee who has got the energy of a small hill, we've got Simon Bridges - the only person under 80 who still buys Brylcreem, and Judith Collins, look her stare caused that ice shelf in Antarctica to crack off and float away."

Davis also compared the National Party to dandruff, and compared Deputy Prime Minister Paula Bennett to Lynn of Tawa, the 1970s character invented by comedian Ginette McDonald.

English said he wasn't offended by the comments but they were negative and cynical. Transport Minister Simon Bridges added his own rebuttal: "I don't use Brylcreem. I'm just saying".

The National Party also responded to Davis' rock insult on Twitter, posting a photo of English with the caption, "this 'rock' got us through the global financial crisis".

Speaking after her own transport announcement, Ardern was asked how Davis' comments squared with her vow to be relentlessly positive.

"Kelvin is a straight talking MP. We are focused on our campaign and running a positive campaign. But at the same time, Kelvin is his own person - I'm not here to stifle his views. We are a team," she said.

"He is a free talker and a straight talker...you wouldn't want us to be one homogeneous bunch."

Davis told the Herald he was mocking, not attacking, the National MPs, and he did so to underline the contrast between "boring, staid" National and Ardern.

He said Ardern didn't know he was going to make the comments, which he said weren't meant to be nasty.

"When they make fun of us, I often think, 'that is quite witty'. It's all a bit of fun."